Do we care about the destruction of sixth century BCE tombs in the Republic of Macedonia to supply antiquities for, say, private collectors?

Yes.

Not because of "nationalist retentionist cultural property laws" (though I could understand a call by archaeologists working in the Republic of Macedonia for the return of specific pieces) but because looting is destroying some unique and highly significant archaeological contexts—and that destruction is removing part of human knowledge for ever.

Looting has intellectual consequences.

Looking at Cuno's article directly, it seems to me that the following passage offers archaeologists flawed advice on the basis of an uneven conception of the role of national sovereignty:

Archaeologists should work with museums to counter the nationalist basis of laws, conventions and agreements, and promote a principle of shared stewardship of our common heritage. Together we should call attention to the failure of these laws to protect our common ancient heritage and perversion of that heritage by claiming the archaeological record as a modern nation’s cultural property[.]

For a modern nation state to claim the archaeological record as cultural property is not a perversion but rather a commonplace application long-standing claims of national prerogative. I use the historical term "Westphalian Sovereignty" to highlight one source of this prerogative: the set of treaties that brought an end to many decades of European war in 1648. Historical debates about the origins of our current international system notwithstanding, nations have since at least that time acted either individually or collectively to address issues of global concern. The system is imperfect, to be sure, but has so far allowed us all to avoid global armageddon while holding out hope for general progress and improvement.

Why then are we asked to carve out an exception when it comes to cultural property and to offer a safe haven from national sovereignty to those who would participate, whether directly or indirectly, in the evident and ongoing looting of global cultural heritage? This makes no sense to me.

As an aside, I am reminded of the time I was a delegate at the 2006 ACLS annual meeting in Philadelphia. During the program, the then Deputy Chairman of the NEH spoke and offered similar arguments about the disjunction between national sovereignty and universal cultural heritage. I was struck by the incongruence of a public servant - an individual whose salary was paid because the sovereign government of the United States chose to compel its citizens both to pay taxes and to take on common debt - standing in front of the American Council of Learned Societies - an organization explicitly organized along national lines - in order to question a state such as Italy's choice to exercise its sovereignty in the protection of material property inside its national boundaries. That seemed odd at best.

To return to Cuno's article, arguments that rely on a unique safe-haven from national sovereignty for the trade in antiquities should be suspect as either arbitrary, self-serving or both.